How do I know if my grapes are ripe, or what’s a degree day?

by Michael GelvenJuly 25, 2017

Planting and managing a vineyard to harvest the best grapes possible is hard work. There is an old saying in the wine industry, “the best wine is produced in the vineyard.” To get the raw material to produce the “best wine possible” requires the vineyard manager to use a combination of old time seat of your pants, common sense and scientific analysis of temperature, rainfall, humidity, and a number of additional factors.

One of these factors is a method known as “Degree Days” (also known as Growing Degree Days or GDD). Degree days is one of the oldest and most accepted methods to allow a vineyard manager to estimate when grapes will be ripe. Degree days are utilized in estimating the harvest of many agricultural crops.

So what’s a degree day? Agricultural crops begin to ripen at a specific temperature, different crops enter the ripening cycle at different temperatures. For example, sweet corn begins to ripen at 45 degrees Fahrenheit while grapes begin to ripen at 50 degree Fahrenheit. To calculate the GDD for grapes you add the day’s low temperature to the day’s high temperature and divide my 2, this gives you the average temperature for the day, then you subtract 50. If this number is a positive number then you add this number to the degree day summary. If this sounds confusing let me give you an example to illustrate; low temperature for the day is 45 degrees high temperature is 95, we add those together and divide by 2 and that comes to 70. We subtract 50 and come to the result that on this particular day we accumulated 20 GDD. We do this day after day or if we are lucky we use a computer that does this each day for us.

Every grape variety has its ripening cycle and the average number of degree days for every variety to reach ripeness is known. Vineyard managers plant a particular grape variety based on the average number of degree days that occur in a particular area. In California these degree day areas are classified as growing regions and certain grape varieties are recommended based on these regions.

Here is a list of the major degree day regions and the recommended varieties to plant in each one:

One interesting facts is that within any geographic area that is classified there can be microclimates that are a different classification. In Napa which is primarily a Region 2 area Carneros which is at the southern end of the valley is a Region 1 and produces outstanding Pinot Noir and Chardonnay.

Below is a chart of degree days for 2017 compared with 2016 in the Napa Valley. Note how many differing areas are included in just the Napa Valley as well as the different micro-climates.